1. Field of the Invention
This invention relates to an electromagnetic identification and location system, comprising at least one interrogator which, in operation, generates an electromagnetic interrogation field by means of at least one antenna coil; at least one responder which in response to the electromagnetic interrogation field generates a code signal in accordance with a binary code stored in a code circuit of the responder, which code signal can be recognized by detection means.
2. Description of Related Art
An electromagnetic identification system with an interrogator capable of generating an interrogation field and a plurality of responders comprising a code circuit in which a unique code is stored for each responder or group of responders, and in response to an interrogation field generating a coded signal which corresponds with the code recorded in the code circuit is known per se.
Such a system, and in particular a responder for such a system are described in Netherlands patent application No. 77,11891. The responder described in the Netherlands application is of the passive type, that is to say, the supply voltage required for the active parts of the responder circuit is derived from the interrogation field. For this purpose the responder comprises a receiver circuit attuned to the interrogation field, in which an A.C. voltage is induced by the interrogation field, which voltage, after being rectified, is supplied to the input terminals of the active circuits of the responder. The active circuits are one or more digital circuits which together form a code generator which, in operation, provides a binary code signal controlling a switch means. The switch means is in turn connected to the receiver circuit, in order to vary the resonance frequency of the receiver circuit and/or the damping of the receiver circuit in accordance with the rhythm of the code. The effect of this can be detected by a detector at the side of the interrogator as a variation of the load of the transmitting antenna. The detector may alternatively be a separate receiver.
The clock pulses required for the operation of the digital circuits are formed by a pulse generator, starting from the A.C. voltage signal in the receiver circuit. In the simplest case, the receiver circuit itself is the pulse generator, the A.C. signal being supplied via a resistor and/or a diode to the clock pulse input(s) of the digital circuits. The clock pulses are then of sinusoidal shape. Often, however, at least one pulse former is used, which converts the sinusoidal signals into signals having steeper edges. Such a pulse former may comprise a single transistor, which is provided with supply voltage in the same way as are the digital circuits.
The pulse generator may alternatively comprise a frequency divider or frequency multiplier, or an oscillator. In those cases, the clock pulse frequency differs from the frequency of the interrogation field.
A known field of application for such an identification system is animal husbandry where cows and other animals have to be recognized, for example, for selectively administering feed. A second field of application is access control, in which individuals entitled to enter carry an electronic label (responder) in the form of a badge, with the doors of the spaces to be safeguarded being opened by the detector, sometimes referred to as reader unit, after recognizing the code of the label. As the label is read at some distance (70 cm) from the door or wall, and the bearer of the label need not perform any separate action to gain access, such a system is referred to as a so-called "hands-free" system.
Another possible field of application resides in the automation of production processes. This is concerned with recognizing and controlling products and product carriers, or tools or tool carriers. An example is the Automatic Guided Vehicle System (AGV systems).
In such AGV systems, it is not only of importance to recognize the product carriers and/or tool carriers, in general the vehicles or carts, but location of the vehicle recognized is also desirable. In other cases, it is not the vehicle which must be identified, but the specific area in which the vehicle finds itself, as well as the exact location of the vehicle within the area identified.